VI.] ENTRY OF PLANT FOODS BY OSMOSIS 157 



The method which is now very largely employed 

 to determine the mineral plant food in the soil that 

 may be regarded as immediately "available" for the 

 crop, consists in attacking the soil with a very dilute 

 acid, whose action shall be comparable with the natural 

 solution processes bringing nutriment to the plant. The 

 mineral matter finds it way by osmosis into the plant 

 in two ways : either from the natural soil water, or from 

 the more concentrated solution formed in immediate 

 proximity to the root-hairs by the attack of the excreted 

 carbon dioxide upon the soil particles. 



The natural soil water is constantly dissolving 

 small quantities of phosphoric acid, potash, and other 

 materials, in which it is aided by the carbonic acid 

 it also contains ; as this water passes by osmosis into 

 the root-hairs it will carry with it the dissolved 

 material, with the exception of any particular ion or 

 radicle which has already attained in the cell sap a 

 higher concentration than it possesses in the external 

 soil solution. But if the soil water alone brought the 

 mineral matter with it, not enough enters the plant 

 to account for the observed facts. For example, the 

 growth of a crop of a ton and a half of clover hay 

 requires the transpiration through the leaves, and 

 therefore the absorption at the root, of about 400 

 tons of water (see p. 91) ; the same crop would also contain 

 about 50 lbs. of potash. If, then, the crop derived all 

 its mineral matter by the simple inflow of the soil 

 water into the root, the 50 lbs. of potash must have 

 been originally dissolved in the 400 tons of water 

 that passed through the crop, which means that the 

 soil water contained as much as 0006 per cent, of 

 potash, a greater concentration than is observed in 

 humid climates. In fact, the particular ions or radicles 

 concerned in nutrition enter the root faster than the 



